As an on-board visual assistant system, a head-up display (HUD) may be used to project vehicle state information (e.g., speed and fuel capacity) and road condition information (e.g., navigation and hazard warning) to an appropriate position in front of a driver, so as to prevent the road ahead from being out of the driver's sight while the driver is viewing the above-mentioned information, thereby to prevent the occurrence of a visual blind area for the driver when the driver is looking down at an instrument board, and improve the driving security.
In the related art, a HUD device including a phase spatial light modulator has already been proposed. However, this kind of HUD device is merely capable of providing one projection plane for the vehicle state information and the road condition information. When both the vehicle state information and the road condition information are to be displayed, they need to be displayed in a same projection plane in a mixed manner, so a display effect may be adversely affected.